1. Field of the Invention
This invention generally relates to methods of forming an optical waveguide for an environmental sensor, and is specifically concerned with a preform for forming a hollow-core, slotted photonic band-gap (PBG) optical fiber for use in an environmental sensor, and methods of forming such a fiber using the preform.
2. Technical Background of the Invention
Environmental sensors in the form of optical fibers having a hollow core are known in the prior art. The hollow core of the fibers used for such sensors typically conducts light by way of a photonic band-gap (PBG) structure surrounding the hollow core. The PBG structure gives rise to a “forbidden frequency range” that corresponds to the wavelength of the light transmitted through the fiber, although hollow core fibers that conduct light via total internal reflection (TIR) for a specific range of wavelengths are also known. Such sensors are used to sense the presence of a “target substance,” such as a particular gas or liquid in the ambient environment. The target substance may be, for example, a threshold amount of carbon dioxide in the ambient air that may be indicative of a fire or other hazardous conditions.
In one prior art design, the hollow core of the optical fiber is exposed to the ambient atmosphere at one or both ends of the fiber to allow gases from the ambient atmosphere to continuously flow into the hollow core. In operation, laser light having a wavelength absorbed by the particular gas composition to be detected is continuously conducted through the hollow core of the fiber. When such a gas is introduced into the open end(s) of the fiber from the ambient atmosphere, it begins to flow through the hollow core. The amplitude of the laser light transmitted through the core diminishes because the gas absorbs the light. In the case of the carbon dioxide example referred to earlier, a certain threshold attenuation may be used to generate a signal that triggers a fire alarm circuit.
Such environmental sensors may be used to detect a broad variety of target substances such as different gas compositions in the atmosphere, organic and inorganic particulates or vapor droplets, and even different liquid compositions when the fiber is immersed in a liquid. Such sensors therefore have broad applicability not only as detectors of combustion products or polluting or potentially toxic substances, but also as control or monitoring sensors in industrial manufacturing processes that require the control of the composition of a particular gas or liquid.
An optical waveguide environmental sensor that includes an elongated side opening that directly exposes the hollow core to the ambient environment is disclosed in U.S. Pat. No. 7,343,074, entitled “Optical waveguide environmental sensor and method of manufacture,” which patent is commonly assigned and incorporated by reference herein. The elongated side opening forms what is referred to in the art as an “optical ridge waveguide.” An optical ridge waveguide typically includes a narrow ridge of material (or materials) that sits on top of a slab of a second (or same) material. A third material of a lower refractive index (often air) surrounds the ridge and the top surface of the slab, thereby providing the refractive guiding mechanism. The slab is typically about the same thickness as the ridge and sits on a substrate of lower refractive index that provides additional vertical confinement.
Optical ridge waveguides are attractive because of their ease of fabrication. The material structure is fabricated in planar layers, and then the waveguide is defined by removing layers of material in selected regions, leaving behind ridges of the original structure. Most ridge waveguides are planar devices that are defined lithographically and then etched using techniques well known in the semiconductor industry. However, it is far more difficult to form such a structure in optical-fiber-based waveguides, particularly in connection with fiber-based environmental sensors that rely on an elongated side opening to expose the hollow core of the fiber to the ambient environment. For example, one way to form an elongated side opening or “slot” in an optical fiber is to use laser machining. Unfortunately, when a laser is used to cut a slot into the fiber, it can adversely affect the strength of the fiber. In addition, forming a long slot using laser machining or mechanical machining is a difficult task.
What is needed, therefore, is a method of forming a hollow-core, slotted PBG optical fiber that does not require machining to form the slot after the fiber is drawn.